Heat sealable and peelable films (also referred to herein as “peelable seals’) are employed on a large scale for temporarily closing containers that include, for example, food products or medical devices. During use, a consumer tears away the peelable film. To gain consumer acceptance, a number of characteristics associated with a heat sealable and peelable film are desired. For example, the film should provide a leak-proof closure of the container or bag. To seal a bag, heat sealing is commonly used. Various apparatus have been constructed for the purpose of forming bags while simultaneously filling the bags with the desired contents. These apparatus are commonly known as vertical form-fill-and-seal and horizontal form-fill-and-seal machines.
These machines typically have forming collars or bars that shape a flat piece of film into a tubular shape of a bag. Hot metal sealing jaws are moved from an open position to a closed position, contacting the film in order to seal it into a bag shape. During the sealing process, the outer layer of the film comes into direct contact with the hot metal surface of the sealing jaws. Heat is thus transferred through the outer layer of the film to melt and fuse the inner sealant layer to form a seal. Generally, the outer layer has a higher melting temperature than the inner sealant layer. As such, while the inner sealant layer is melted to form a seal, the outer layer of the film does not melt and is not stuck to the sealing jaws. After the sealing jaws reopen, the film is cooled to room temperature.
Before the inner sealant layer is cooled to room temperature, it should be able to maintain its seal integrity. The ability of an adhesive or sealant layer to resist creep of the seal while it is still in a warm or molten state is generally referred to as “hot tack.” To form a good seal, the hot tack of the sealable and peelable film should be adequate.
Besides adequate hot tack, it is also desirable to have a low heat seal initiation temperature which helps to ensure fast packaging line speeds and a broad sealing window which could accommodate variability in process conditions, such as pressure and temperature. A broad sealing window also enables high speed packaging of heat sensitive products, as well as, provides a degree of forgiveness for changes in packaging or filling speeds.
In addition to the “sealable” characteristic of a sealable and peelable film, it should also have a desired “peelable” characteristic needed to provide an easily openable seal on a package or bag. Peelability generally refers to the ability to separate two materials or substrates in the course of opening a package without compromising the integrity of either of the two. The force required to pull a seal apart is called “seal strength” or “heat seal strength” which can be measured in accordance with ASTM F88-94. The desired seal strength varies according to specific end user applications. For flexible packaging applications, such as cereal liners, snack food packages, cracker tubes and cake mix liners, the seal strength desired is generally in the range of about 1-4 pounds per inch. For example, for easy-open cereal box liners, a seal strength in the range of about 1-3 pounds per inch is commonly specified, although specific targets vary according to individual manufactures requirements. In addition to flexible packaging application, a sealable and peelable film can also be used in rigid package applications, such as lids for convenience items (e.g., snack food such as puddings) and medical devices. Typical rigid packages have a seal strength of about 1-5 pounds per inch. The seal layer can be on the lid or on the container or both.
Additional desired characteristics for a heat sealable and peelable film include a low coefficient of friction and good abuse resistance. A low coefficient of friction ensures that the sealant layer can be processed smoothly and efficiently on fabrication and packaging equipment and is particularly important for vertical form-fill-and-seal packaging. Good abuse resistance and toughness is desired, for example, in cereal box liners to withstand tears and punctures from irregularly-shaped, rigid cereals. Additional characteristics include taste and odor performance and barrier or transmission properties.
In some applications, it is important to be able to identify when the package has been unsealed as, for example, it may indicate when a package has been the subject of tampering.
Heat sealable and peelable films are generally made from one or more polymeric resins. The resulting characteristics of a heat sealable and peelable film depend largely upon the type of the resins used to form the film. For example, ethylene vinyl acetate (EVA) and ethylene methyl acrylate (EMA) copolymers provide excellent heat sealing properties. However, the seals produced with these copolymers are such that separation usually cannot be achieved without damage to the film. To alleviate this problem, polybutylene is mixed with an EVA polymer to produce a heat sealable and peelable film. Although the peelability of the film is improved, the heat sealable and peelable film has some unpleasant odor due to the presence of EVA. In addition to using polybutylene, some ionomers, such as SURLYN®, is mixed with EVA to produce a heat sealable and peelable film. While the film is peelable, it causes stringiness or “angel hair” upon separation of the film. Moreover, ionomers are generally expensive and may have some odor as well.
Additionally, typical EVA-polybutylene based peelable seal systems “age up” in seal strength. The seal strength increases as time elapses after seal formation. This is seen as a disadvantage of these systems because seals that are secure at the time the package is formed increase in strength prior to reaching the consumer, making the packages more difficult for the consumer to open.
U.S. Pat. No. 6,590,034 describes peelable seals made from a mixture of two immiscible polymers which form a continuous phase and a discontinuous phase wherein the absolute value of the shear viscosity differential of the two polymers is less than 100%. Although many potential materials are covered, this reference focuses on the use of homopolymer polypropylene as the discontinuous phase.
Although a number of resins systems have been employed to make a heat sealable and peelable film, there continues to exist a need for an improved cost-effective heat sealable and peelable film with desired seal strength during processing and transportation as well as during package opening by the end consumer. It is desirable that the resin system used to produce the heat sealable and peelable film has a relatively lower seal initiation temperature and a relatively broad heat sealing window. It is also desirable that the heat sealable and peelable film is relatively age-resistant and has a relatively lower coefficient of friction and good abuse resistance and toughness.
U.S. Pat. No. 7,863,383 describes blends of from about 5 to about 98 percent by weight propylene based elastomers or plastomers with a particular second polymer from a group preferably consisting of polyethylene and styrenic polymers, which would have a seal strength in the range that would make them particularly well suited for use as a peelable seal. It would be desirable to have further improved resins for use in peelable seal applications.